The drum used, for example, with a batch-type asphalt plant as an aggregate drier or as a mixer for making asphaltic concrete, is subject to very high temperatures in the area where the burner flame enters the drum for drying the virgin aggregate. Typical drum surface temperatures in this area may be 650.degree. to 750.degree. F. or so which is destructive to the drum itself as well as wasting heat by virtue of radiation from the drum wall. When a drum mixer is used for recycling used asphaltic concrete, in which the latter is introduced midway or so down the drum, the wall temperatures of the upstream portion of the drum are usually even higher, 750.degree. to 850.degree. F. or so, since less virgin aggregate is introduced adjacent the burner. The chief object of the present invention is to reduce that heat loss and the concomitant deleterious heating of the drum wall.